Closely related documents include coowned U.S. Pat. No. 4,963,882 entitled xe2x80x9cPRINTING OF PIXEL LOCATIONS BY AN INK JET PRINTER USING MULTIPLE NOZZLES FOR EACH PIXEL OR PIXEL ROWxe2x80x9d, U.S. Pat. No. 4,965,593 entitled xe2x80x9cPRINT QUALITY OF DOT PRINTERSxe2x80x9d, U.S. Pat. No. 5,555,006 entitled xe2x80x9cINKJET PRINTING: MASK-ROTATION-ONLY AT PAGE EXTREMES; MULTIPASS MODES FOR QUALITY AND THROUGHPUT ON PLASTIC MEDIAxe2x80x9d, and U.S. Pat. No. 5,561,449, entitled xe2x80x9cPOSITION LEADING, DELAY, and TIMING UNCERTAINTY TO IMPROVE POSITION and QUALITY IN BIDIRECTIONAL INKJET PRINTINGxe2x80x9d; as well as U.S. patent application Ser. No. 08/667,532, entitled xe2x80x9cJITTER-FORM BACKGROUND CONTROL FOR MINIMIZING SPURIOUS GRAY CAST IN SCANNED IMAGESxe2x80x9d and now issued as U.S. Pat. No. 5,859,928, and two additional U.S. patent applications filed generally concurrently herewith having these Hewlett Packard Company later identified as Ser. No. 08/814,949 and issued as U.S. Pat. No. 6,082,849, entitled xe2x80x9cRANDOM PRINTMASKS IN A MULTILEVEL INKJET PRINTERxe2x80x9d, and later identified as Ser. No. 08/810,747 and issued as U.S. Pat. No. 6,250,739, entitled xe2x80x9cBIDIRECTIONAL COLOR PRINTMODES WITH SEMISTAGGERED SWATHS TO MINIMIZE HUE SHIFT AND OTHER ARTIFACTSxe2x80x9d. All these documents in their entireties are hereby incorporated by reference into this document.
This invention relates generally to machines and procedures for printing ultrahigh-resolution color text or graphics on printing media such as paper, transparency stock, or other glossy media; and more particularly to a scanning inkjet machine and method that construct text or images from individual ink spots created on a printing medium, in a two-dimensional pixel array. The invention employs print-mode techniques to optimize ultrahigh-resolution color image quality vs. operating time.
A previous generation of printing machines and procedures has focused on mixed resolution. These systems most typically have employed about 24 pixels/mm (600 pixel dots per inch, or xe2x80x9cdpixe2x80x9d) in a carriage scan direction transverse to the printing medium and 12 pixels/mm (300 dpi) in the print-medium advance direction longitudinal to the printing mediumxe2x80x94or 24 pixels/mm for black and 12 pixels/mm for chromatic colors, or relatively tall 12 mm (half-inch) pens for black ink and relatively short 8 mm (third-inch) pens for chromatic colors; or combinations of these and other operating-parameter mixtures.
These mixed-resolution systems have been of interest for obtaining effectively very high quality printing with a minimum of developmental delay. In the continuing highly competitive development of inkjet printer products, the mixed systems have served a very important role because of many difficult problems associated with attaining a full ultrahigh resolutionxe2x80x94for example 24 pixel/mm pens, 12 mm tall, for all colorants in a color-printing system.
In the current generation of machines, interest has shifted to solving those many difficult problems. As will be seen, most of such difficulties have been recognized for many years, but tend to be aggravated in the ultra-high-resolution environment.
(a) Throughput and Cost
In a sense many problems flow from these two considerations, since essentially all the problems would evaporate if it did not matter how slow or expensive a printer was. In practice, marketplace pressures have made it crucially important that a printer be both competitively fast (even when printing in a xe2x80x9cqualityxe2x80x9d mode) and competitively economical.
(b) Firing Frequency
Thus for example high throughput in combination with high resolution pushes the capability of economical inkjet nozzles to fire at a high enough repetition rate. An inkjet pen tends to be most stable in operation, and to work best for error hiding, at a low firing frequency.
Horizontal resolution of 24 pixels/mm if printed all in a single pass, however, would require a rather high firing frequencyxe2x80x94in fact, for current-day technology, roughly twice the highest frequency of reliable operation in an economical pen. This figure may be expected to change with refinements in pens.
(c) Banding and Pattern Artifacts
These spurious image elements are well known in lower-performance printers, but like other problems can be even more troublesome in the newer generation of devices. It is known, for example, that some banding effects can be reduced by printing highly staggered (i. e., overlapping) swathsxe2x80x94but also that doing so reduces overall throughput proportionately. (A different kind of visible banding, associated with hue shifts, will be discussed below.) Hence, again, high throughput tends to run counter to elimination of banding, and this conflict is aggravated by a requirement for printing at resolution that is twice as fine.
As to pattern defects, the design of dither arrays is a logical culprit and has previously received a great deal of attention in this regard, and may be considered highly refined. Yet heretofore some patterning persists in high-resolution images printed under conditions which should yield the best possible image quality.
Theory suggests that no further advantage can be obtained through dither redesign, and that solutions must be sought elsewhere. Discussion of printmasks in a following subsection of this document will take up this theme again.
Generally speaking, tools for investigating this area heretofore have been inadequate.
(d) Color shift
One important approach to maximizing throughput is to print bidirectionally. In a bidirectional-printing system the pens print while the carriage is traveling in each of its two directionsxe2x80x94i. e., across the printing medium, and back.
This technique is well known and successful for printing in monochrome. Workers skilled in this field have recognized, however, that for printing in color a hue shift, or more precisely a color shift, arises as between printing in the two directions.
The reason is that pens are traditionally arranged, physically, on their carriage in a specific sequence. Therefore if two or more of the pens fire while the carriage is moving in one particular direction the different ink colors are laid down one on top of another in a corresponding orderxe2x80x94and while the carriage is moving in the opposite direction, in the opposite order.
Usually the first inkdrop of two superposed drops tends to dominate the resulting perceived color, so that for example laying down magenta on top of cyan produces a blue which is biased toward the cyan; whereas printing cyan on top of magenta typically yields a blue which emphasizes magenta. If successive separate swathsxe2x80x94or separately visible color bands, subswathsxe2x80x94are printed while the pen is thus traveling in each of two directions, respectively, the successive swaths or subswaths. Banding that results is often very conspicuous.
For this reason, previous artisans have striven to avoid printing of any superposition-formed secondary colors in more than one order, ever. Printers commercially available under the brand names Encad(copyright) and Laser-master(copyright), in particular, employ a tactic that employs brute force to avoid sequence changes: the pens are offset, with respect to the vertical direction, or in other words longitudinally along the printing medium.
They are offset by the full height of each nozzle arrayxe2x80x94posing, at the outset, significant problems of banding (see discussion following) as between colors. Furthermore, in consequence of the full-height-offset arrangement each of the trailing three pens must print over a color subswath formed in at least one previous scanxe2x80x94from one to three previous scans, depending upon which pen is under consideration.
This system advantageously maintains a fixed color sequence even in bidirectional printing. Use of full-height offset of the pens, however, makes a great sacrifice in other operating parameters. More specifically, the full-height staggered pens have a print zone that is four color bands (subswaths) tall.
Necessarily the overall product size in the direction of printing-medium advance is correspondingly greater, as are weight and cost. In addition the extended printzone is more awkward to manage in conjunction with a round (i. e. cylindrical) platen.
Furthermore in this system it is considerably more awkward to hold the printing medium consistently flat and without relative motion. Still further, the trailing pen is overprinting a pixel grid that has already been inked by three preceding pens, and in a heavy-color region of an image this means that a considerable amount of liquid has already been laid down on the page, and the page has had a significant time to deform in response.
Substantial and uncontrollable intercolor registration problems may be expectedxe2x80x94particularly in view of the fact that this liquid-preloading effect is differential as between the several pens. In other words, it is present even for the second pen in the sequence, but suffered with progressively greater severity by the third and fourth.
The Encad/Lasermaster systems use bidirectional printing for at least the so-called xe2x80x9cfastxe2x80x9d and possibly xe2x80x9cnormalxe2x80x9d printing modes, but not for the xe2x80x9cbestxe2x80x9d-quality mode (which prints unidirectionally). Of course use of unidirectional printing as a best-quality printing mode incurs a throughput penalty of a factor as high as two. (Because the retrace may be at a faster, slew speed the factor may be less than two.) Such a penalty can be very significant.
Thus the art has failed to deal effectively with hue shiftsxe2x80x94an impediment to fully exploiting the potential of bidirectional printing as a means of enhancing throughput.
(e) Liquid Loading
Hue shift, however, is not the only problem that is associated with bidirectional printing. Another is microcoalescence. This may be regarded as a special case (particularly afflicting ultrahigh-resolution operation) of excessive inking with its historically known problemsxe2x80x94which are summarized below.
In still another difficulty, the tails or satellites of secondary-color dots, pointing in opposite directions, can generate textural artifacts when the left-to-right order is reversed.
Excessive inking is a more-familiar problem. To achieve vivid colors in inkjet printing with aqueous inks, and to substantially fill the white space between addressable pixel locations, ample quantities of ink must be deposited. Doing so, however, requires subsequent removal of the water basexe2x80x94by evaporation (and, for some printing media, absorption)xe2x80x94and this drying step can be unduly time consuming.
In addition, if a large amount of ink is put down all at substantially the same time, within each section of an image, related adverse bulk-colorant effects arise: so-called xe2x80x9cbleedxe2x80x9d of one color into another (particularly noticeable at color boundaries that should be sharp), xe2x80x9cblockingxe2x80x9d or offset of colorant in one printed image onto the back of an adjacent sheet with consequent sticking of the two sheets together (or of one sheet to pieces of the apparatus or to slipcovers used to protect the imaged sheet), and xe2x80x9ccocklexe2x80x9d or puckering of the printing medium. Various techniques are known for use together to moderate these adverse drying-time effects and bulk- or gross-colorant effects.
(f) Prior Print-mode Techniques
One useful and well-known technique is laying down in each pass of the pen only a fraction of the total ink required in each section of the imagexe2x80x94so that any areas left white in each pass are filled in by one or more later passes. This tends to control bleed, blocking and cockle by reducing the amount of liquid that is all on the page at any given time, and also may facilitate shortening of drying time.
The specific partial-inking pattern employed in each pass, and the way in which these different patterns add up to a single fully inked image, is known as a xe2x80x9cprintmodexe2x80x9d. Heretofore artisans in this field have progressively devised ways to further and further separate the inking in each pass.
Larry W. Lin, in U.S. Pat. No. 4,748,453xe2x80x94assigned to Xerox Corporationxe2x80x94taught use of a simple checkerboard pattern, which for its time was revolutionary in dividing inking for a single image region into two distinct complementary batches. Lin""s system, however, maintains contact between pixels that are neighbors along diagonals and so fails to deal fully with the coalescence problem.
The above-mentioned U.S. Pat. No. 4,965,593, which is in the name of Mark S. Hickman, teaches printing with inkdrops that are separated in every directionxe2x80x94in each printing passxe2x80x94by at least one blank pixel. The Hickman technique, however, accomplishes this by using a nozzle spacing and firing frequency that are multiples of the pixel-grid spacing in the vertical and horizontal directions (i. e., the medium-advance and scan axes respectively).
Accordingly Hickman""s system is not capable of printing at on intervening lines, or in intervening columns, between the spaced-apart inkdrops of his system. This limitation significantly hinders overall throughput, since the opportunity to print such further intervening information in each pass is lost.
Moreover the Hickman system is less versatile. It forfeits the ability to print in the intervening lines and columns even with respect to printmodes in which overinking or coalescence problems are absentxe2x80x94such as, for example, a high-quality single-pass mode for printing black and white text.
The above-mentioned U.S. Pat. No. 5,555,006, which is in the name of Lance Cleveland, teaches forming a printmask as plural diagonal lines that are well separated from one another. Cleveland introduces printmodes that employ plural such masks, so that (unlike Hickman) he is able to fill in between printed elements in a complementary way.
It is certainly not intended to call into question the Cleveland teaching, which represents a very substantial advance in the artxe2x80x94over both Lin and Hickman. Cleveland""s invention, however, in part is aimed at a different set of problems and therefore naturally has only limited impact on general overinking problem discussed here. In particular Cleveland seeks to minimize the conspicuousness of heater-induced deformation at the end of a page.
Thus even Cleveland""s system maintains the drawback of inkdrop coalescence along diagonals and sometimesxe2x80x94since he calls for very steeply angled diagonal lines which in some segments are formed by adjacent vertical pixelsxe2x80x94even along columns.
Another ironic development along these lines is that the attempts to solve liquid-loading problems through printmask tactics in some cases contribute to pattern artifacts. It will be noted that all the printmodes discussed abovexe2x80x94those of Lin, Hickman, Cleveland, and other workers not mentionedxe2x80x94are all highly systematic and thus repetitive.
For example, some printmodes such as square or rectangular checkerboard-like patterns tend to create objectionable moirxc3xa9 effects when frequencies or harmonics generated within the patterns are close to the frequencies or harmonics of interacting subsystems. Such interfering frequencies may arise in dithering subsystems sometimes used to help control the paper advance or the pen speed.
(g) Known Technology of Printmodes
One particularly simple way to divide up a desired amount of ink into more than one pen pass is the checkerboard pattern already mentioned: every other pixel location is printed on one pass, and then the blanks are filled in on the next pass.
To avoid horizontal xe2x80x9cbandingxe2x80x9d problems (and sometimes minimize the moirxc3xa9 patterns) discussed above, a printmode may be constructed so that the printing medium is advanced between each initial-swath scan of the pen and the corresponding fill-swath scan or scans. This can be done in such a way that each pen scan functions in part as an initial-swath scan (for one portion of the printing medium) and in part as a fill-swath scan.
This technique tends to distribute rather than accumulate print-mechanism error which is impossible or expensive to reduce. The result is to minimize the conspicuousness ofxe2x80x94or, in simpler terms, to hidexe2x80x94the error at minimal cost.
The pattern used in printing each nozzle section is known as the xe2x80x9cprintmode maskxe2x80x9d or xe2x80x9cprintmaskxe2x80x9d, or sometimes just xe2x80x9cmaskxe2x80x9d. The term xe2x80x9cprintmodexe2x80x9d is more general, usually encompassing a description of a maskxe2x80x94or several masks, used in a repeated sequence or so-called xe2x80x9crotationxe2x80x9dxe2x80x94and the number of passes required to reach full density, and also the number of drops per pixel defining what is meant by xe2x80x9cfull densityxe2x80x9d.
Operating parameters can be selected in such a way that, in effect, mask rotation occurs even though the pen pattern is consistent over the whole pen array and is never changed between passes. Figuratively speaking this can be regarded as xe2x80x9cautomaticxe2x80x9d rotation or simply xe2x80x9cautorotationxe2x80x9d.
As mentioned above, some of these techniques do help to control the objectionable patterning that arises from the periodic character of printmasks employed heretofore. Nevertheless, for the current new generation of ultrahigh-resolution color printers generally speaking the standards of printing quality are higher, and a more-advanced control of this problem is called for.
(h) Other Limitations
Heretofore true six-hundred-dot-per-inch color printing has not been produced with a single pass of the printheads. One reason is that working inkdrop sizes have been excessive for such fine or ultrafine resolution.
Carriage-scan speeds and pen firing frequencies in some or all cases have been unsuitable. Moreover the number of passes needed to complete each swath or subswath has not been high enough for best quality.
Furthermore it has not been possible heretofore for each printhead to provide color inkdrop placement on every pixel row during a single pass. Finally, allocations of passes to pixels by printmasks has not adequately optimized the operation of printing systems for use in true six-hundred-dot-per-inch printing.
(i) Conclusion
Thus persistent problems of firing frequency, hue shift, liquid loading, and pattern artifacts, countermeasured against pervasive concerns of throughput and cost, have continued to impede achievement of uniformly excellent inkjet printing. It may be added that certain combinations of these difficulties are more readily controlled on one and another printing medium; however, at least some of these problems remain significant with respect to all industrially important printing media.
Thus, as can be seen, important aspects of the technology used in the field of the invention remain amenable to useful refinement.
The present invention introduces such refinement. This inkjet printing system uses high-resolution printheads of different color inks to produce true six-hundred-dot-per-inch color printing with a single pass of the printheads.
By achieving a smaller drop size in the range of twenty-five to fifty picoliters for each printhead using a separate ink color (yellow, cyan, magenta, black), a full range of high-resolution color print-quality modes is possible. In an exemplary embodiment with a carriage scan speed of twenty-five inches per second, a two-pass draft or xe2x80x9cfastxe2x80x9d mode uses a 7.5 kHz firing frequency, a four-pass xe2x80x9cnormalxe2x80x9d mode uses a 4 kHz firing frequency, and an eight-pass xe2x80x9cbestxe2x80x9d or xe2x80x9cbest qualityxe2x80x9d mode uses a 2 kHz firing frequency.
In all of these print modes, each printhead can provide color inkdrop placement on every pixel row during a single pass. All of the foregoing operational principles and advantages of the present invention will be more fully appreciated upon consideration of the following detailed description, with reference to the appended drawings, of which: